1. Technical Field
This invention relates to measuring torsional distortion, and in the preferred embodiment provides a method and device for measuring the distortion in a body which is subject to applied torque. The torsional distortion measured may be used, in known manner, to determine the magnitude of the torque to which the body is subject.
2. Description of Related Art
It has been known for many years to measure the torque to which a body (for example a shaft) is subject by applying one or more strain gauges to the surface of the body and interrogating the output of the strain gauges to provide an indication of torque. In recent years, torque measurement techniques have been substantially improved by the introduction of Surface Acoustic Wave (“SAW”) based torque measurement devices (see for example EP-A-05 18900). Such devices have a number of well recognized advantages including high sensitivity and the ability to provide torque measurements without the need for hard wired connections to the SAW device.
Up until now, the conventional technique for mounting torque measuring devices has consisted of preparing the surface of the body whose torsional distortion is to be measured and securing the torque measuring device to the prepared surface. The preparation may consist of a cleaning process subsequent to which the torque measuring device is secured to the cleaned surface by a bonding technique—for example adhesive bonding or soldering. However, many torque measuring devices require a flat surface upon which to be mounted and accordingly the preparation may include the step of providing a flat surface, for example by machining, on the body whose torsional distortion is to be measured, prior to securing the torque measuring device to the prepared flat. These techniques are well recognized as being disadvantageous because of the time and cost involved and because they limit the possibility of retrofitting a torque measuring device to an existing shaft. Further, the mounting of the torque measuring device requires both skilled labor and well defined manufacturing conditions and is accordingly difficult to carry out under field conditions. As a result it is generally necessary to apply torque measuring devices in a factory environment before the body whose torsional distortion is to be measured is mounted in its final use position. As a result the torque measuring device is susceptible to accidental damage during the period between mounting of the torque measuring device and the final positioning of the body whose torsional distortion is to be measured. Finally, existing techniques require the permanent mounting of the torque measuring device on the body whose torsional distortion is to be measured. As a result, if the torque measuring device fails, substantial additional work will be involved in terms of removing the faulty unit before a replacement unit can be installed.
Accordingly, there exists a need for a method and apparatus for measuring torsional distortion of a body which may easily be applied to the body without extensive preparation of the body. The need also arises for such a device which can be applied under field as well as factory conditions and which can be applied by skilled, semi-skilled and unskilled labor. A need also arises for such a device which can readily be removed and replaced in the event that it becomes faulty.